1. Field of the Invention
The present invention relates to ultrasonic transducer devices that are included in ultrasound scopes, in ultrasound miniature probes, and in ultrasound capsule endoscopes, and that are manufactured by using the micromachining processes, and also relates to a method of controlling the same.
2. Description of the Related Art
The diagnostic ultrasound method, in which ultrasound is emitted onto walls of body cavities and the state of the body is visualized on the basis of the signals echoed from the walls for making a diagnosis, is widely used. One of the instruments used for the diagnostic ultrasound method is an ultrasound endoscope scope.
An ultrasound endoscope scope has an ultrasound probe at the distal end of the insertion tube that is to be inserted into body cavities. The ultrasound probe converts electric signals into ultrasound to emit the ultrasound onto body cavities, and receives the ultrasound reflected in the body cavities in order to convert the received ultrasound into electric signals.
Conventionally, for manufacturing ultrasound probes, piezoelectric ceramics (PZT: lead-zirconate-titanate) are used as the piezoelectric device that is used for converting electric signals into ultrasound.
In recent years, capacitive micromachined ultrasonic transducers (hereinafter referred to as cMUTs) that are obtained by processing a silicon semiconductor substrate have received attention. A cMUT is one of several MEMS (Micro Electro-Mechanical System) devices.
A diagnosis modality called harmonic imaging has attracted lots of attention because it permits an ultrasonic diagnosis with a high level of accuracy that the conventional methods have never been able to attain. Accordingly, it has become essential for the body-cavity-insertion-diagnostic ultrasound systems to be compatible with this diagnosis modality. Because of this, it is desirable that ultrasonic transducers have broader bandwidths.
As is mentioned above, cMUTs that are manufactured using micromachining processes have recently received attention. The merit of cMUTs is not only that they do not contain heavy metals such as lead, but also that wide bandwidth characteristics can easily be achieved. Accordingly, cMUTs are suitable for harmonic imaging.
FIG. 1 shows an example of a conventional cMUT. The cMUT shown in FIG. 1 is the cMUT disclosed in WO 2001/097562. This ultrasonic transducer is constituted of a plurality of cMUTs. Respective cells that constitute each cMUT have a charged membrane 206. This charged membrane 206 has capacitance and is opposed to a substrate 205 that is inversely charged.
This charged membrane 206 is curved by the bias charging in the direction of the substrate 205. Also, the substrate 205 has a center that is elevated in such a manner that the elevated portion gets closer to the center of the charged membrane 206 and the density of the charged particles becomes the highest around the center of the vibration of the charged membrane 206. For the purpose of realizing the operations by using harmonics, the driving pulse waveform provided for the cells are distorted in advance. This is because the non-linear operations of devices are considered in order to reduce the harmonic components that may be generated in transmission ultrasound by a driving signal that does not have distortion.
cMUT cells can be integrated with a transducer controlling circuit such as a bias charging regulator 201 because the cMUT cells are processed by using conventional semiconductor processing methods. The cMUT cells can be processed also by using micro-stereolithography. Accordingly, the cells are formed by using various materials such as polymers and the like.
The above mentioned diagnostic ultrasound system has a high voltage-proof switch in the ultrasound probes so that it can operate at a high voltage. The diagnostic ultrasound system has a pulse generation unit and a control unit. The pulse generation unit can output pulses that have any waveforms or any voltage values. The control unit controls the output from the above high voltage-proof switch and pulse generation unit on the basis of the scanning timing of the ultrasonic transducer.
In view of the circumstances above, the present applicants suggested a method in which the DC voltage is applied only at a timing that corresponds to the application of the rf signal (Japanese Patent Application Publication No. 2004-176039).
FIG. 2 shows a first example of a method of driving an ultrasonic transducer; and this method is employed in the conventional piezoelectric transducer driving techniques. The example shown in FIG. 2 is a probe that is disclosed in Japanese Examined Patent Application No. 63-026341. This probe includes, in addition to the known circuits, an additional operation circuit in order to minimize the influence of electrical interference that can be caused by a cable that is used for connecting the probe and the ultrasound signal evaluation device and that is relatively long. In the technique disclosed in Japanese Examined Patent Application No. 63-026341, the probe includes the above described circuit; however, the circuit is not so large. Also, the operations for the ultrasound inspection are not so difficult.
The probe housing of the probe includes a transmission circuit 210. The transmission circuit 210 includes a booster coil 211, a VMOS field effect transistor (VMOS FET) 213, a control circuit 214, and a capacitor 215. The VMOS FET 213 is turned on and off in accordance with a control signal 212.
The operations of the transmission circuit 210 are explained hereinbelow. First, the capacitor 215 is charged at a high density via the booster coil 211. When the amount of the charge in the capacitor 215 reaches the upper limit, a control signal is output from the control circuit 214 to the switch driving terminal in the VMOS FET 213. Then, the VMOS FET 213 enters an ON state. Then, the discharging starts in the closed circuit of the ON state resistance, a resistor 216, and the capacitor 215. The voltage generated by this discharged current in the resistor 216 is applied to a piezoelectric transducer.
However, when a high voltage is to be induced by using this method, the booster coil 211 requires a high inductance. If the booster coil 211 has a high inductance, a resonance is caused by the capacitor 215 and the booster coil 211, and the driving pulse comes to include ringing. This ringing signal is applied to the piezoelectric transducer without being reduced or blocked, which causes deterioration in the spatial resolution and the S/N ratio.
FIGS. 3A and 3B show a second example of a method of driving a piezoelectric ultrasonic transducer that is employed in the conventional techniques. FIG. 3A shows a diagnostic ultrasound system disclosed in Japanese Patent No. 3062313. FIG. 3B shows the same system in a simplified manner. Japanese Patent No. 3062313 discloses a configuration for minimizing the influence of the electric interference caused by a long connection cable although the technique disclosed in this document is not intended to have a countermeasure against the above ringing.
In FIGS. 3A and 3B, an ultrasound probe 220 and a diagnostic ultrasound system 221 are shown. Ultrasound signals are emitted and received by an ultrasonic transducer 222 provided in the ultrasound probe in order to perform the ultrasound scan on the subject. The diagnostic ultrasound system 221 can obtain an ultrasound sectional image on the basis of the received ultrasound signals.
In the ultrasound probe 220, a high voltage-proof switch 223 is provided. In the above diagnostic ultrasound system 221, a pulse generation unit 227 and a control unit 228 are provided. The pulse generation unit 227 can output a pulse that has any voltage value in any waveform. The control unit 228 controls the output of the voltage-proof switches 223 and the pulse generation unit 227 in accordance with the timing of the scan performed by the ultrasonic transducer.
By the above configuration, the size of the electric circuit in the ultrasound probe is reduced. Also, high voltage pulse signals for driving the ultrasonic transducer can be generated efficiently in the probe. Also, excellent ultrasound images that are not influenced by the interference caused by the cable can be obtained, and noise that leaks to the external environment can be reduced. Also, the ringing is not caused because there is no element that can cause resonation in the circuit.
As described above, it is proposed to produce a micro piezoelectric transformer and a micro electromagnetic transformer and to arrange them close to a cMUT and a pMUT (a piezoelectric transducer that is produced by using the micromachining processes).